Photochromic spectacles are spectacles which lenses are quickly colored in an outdoor irradiated with light containing UV rays such as sunlight to function as sunglasses and which are discolored in an indoor irradiated with no such light to function as ordinary transparent spectacles, and demands thereto grow larger in recent years.
Known processes for producing plastic lenses having a photochromic property are a process in which lenses having no photochromic property are impregnated on surfaces with photochromic compounds (hereinafter referred to as an imbibition process), a process in which a photochromic compound is dissolved in a monomer and the monomer is polymerized to thereby directly obtain photochromic lenses (hereinafter referred to as an in mass process) and a process in which a coating agent (hereinafter referred to as a photochromic coating agent) comprising a photochromic compound is applied on the surfaces of plastic lenses to provide a layer (photochromic coat layer) having a photochromic property (hereinafter referred to as a coating process). Among these processes, the coating process has an advantage in that it can provide photochromic property to any lens base materials in principle, as compared with the other two processes. Besides, a soft base material in which a photochromic compound is liable to be dispersed has to be used as a base material for lenses in the imbibition process, and a specific composition has to be used as well in the in mass process in order to allow the good photochromic property to be revealed. However, the coating process does not have such restriction regarding base materials.
Examples of the photochromic coating agents used in the coating process may include a photo-polymerizable one (as disclosed in the pamphlet of WO-2001/02449). The present inventors also developed a photo-curable photochromic coating agent capable of providing a photochromic coating layer having excellent adhesion to a base material (as disclosed in the pamphlet of WO-2003/011967, Japanese Patent Application Nos. 2002-354291 and 2002-372835).
In order to form a photochromic coat layer using the photo-polymerizable photochromic coating agent, the coating agent is applied on the surface of a lens base material and thereafter, the coated film of the coating agent may be cured by irradiation with ultraviolet ray or other lights. However, it is not said still that the technique relating to the coating process has been accomplished. Even in the case of carrying out the curing as described above, a photo-curing apparatus for hard coating has been used.
As an apparatus of this kind, an apparatus capable of automatically carrying out a series of treatments from application of a coating agent to photo-polymerization is known. The apparatus has such a structure that a lens base material coated with the coating agent is automatically transported to a curing station where a light source for ultraviolet ray is equipped inside a cover and ultraviolet ray irradiation can be carried out while nitrogen gas is introduced into the cover (referred to JP-A-2000-334369).
It is considered that the coating process with the above apparatus may be a usable process as the process for preparing a photochromic plastic lens, however it has been found to have such problem that it is difficult to prepare a high quality photochromic coat layer by practically carrying out the coating with the photochromic coating agent. As one of the reasons, it is considered that the polymerization rate is lowered by light absorption of the photochromic compound contained in the coating agent and thereby it easily suffers polymerization inhibition due to residual oxygen in the polymerization atmosphere. In the case of preparing a lens having sufficient photochromic properties with the coating process, the thickness of the coat layer is necessary to be very thicker as compared with the thickness of the hard coat film (usually about 1 μm), for example, thicker than 5 μm, preferably thicker than 30 μm, since the concentration of the photochromic compound in the photochromic coating agent has a limit. Therefore, as another of the reasons, it is considered when the thickness of the layer is made to be large, the surface of the layer easily suffers the influence of polymerization hindrance caused by oxygen, while the deep part thereof slightly suffers the influence and thereby the polymerization degree is uneven in the thickness direction. Therefore, it is considered that the above problems may be solved by increasing the polymerization rate or carrying out the polymerization in an atmosphere substantially free from oxygen.
However, it is not preferred to increase the amount of a photo-polymerization initiator for preventing the polymerization rate from lowering because of inducing such a problem of lowering the durability of the photochromic compound. Furthermore, in the case of avoiding the problem of polymerization inhibition caused due to oxygen by sufficiently purging the atmosphere with an inert gas such as nitrogen gas, it takes a long period of time to lower the oxygen concentration into an unproblematic level by purging with an inert gas after disposing a base material coated with a coating agent in a polymerization apparatus. As the result, it is further found that the case of using a base material having a convex or concave curved surface such as lenses for spectacles causes different problems that even if the base material is horizontally stood, it is impossible to prepare a coating layer having an uniform and prescribed thickness because the coating agent is fluid, and the occurrence of color unevenness at the time of coloring and the lowering of optical properties of lenses are inevitable.
Under the circumstances, it is an object of the present invention to provide a process for forming a coat layer of a cured material made of a coating agent on the surface of a base material having a convex or concave curved surface such as lenses for spectacles or the like, and also to provide a process for uniformly and homogeneously curing a coating agent layer even if having a large thickness in an uncured state.
The present inventors have earnestly studied in order to solve the above problems. As a result, the present inventors have succeeded to carry out polymerization in a desired atmosphere for a very short period of time from the completion of the coating by using a photo-polymerization apparatus in which, in addition to a photo-polymerization chamber for carrying out photo-polymerization, a polymerization preliminary chamber is provided as a preceding chamber of the photo-polymerization chamber, while previously regulating the atmospheres of the both chambers, a base material coated with a photo-curable coating agent is once held in the polymerization preliminary chamber and then transferred to the photo-polymerization chamber by a specific transporting means. Furthermore, using the apparatus, various conditions for preparing the favorable coat layer have been variously studied and succeeded to specify the oxygen concentration capable of ignoring the influence of polymerization inhibition due to oxygen and the time capable of ignoring the deformation during a period of from the time just after the coating film formation to the time carrying out the polymerization curing. Thus, the present invention has been accomplished.
As a method for applying the coating agent on the surface of a lens base material, a method of using a spin coat apparatus (spin coat method) is known. The spin coat method comprises the steps of feeding a coat liquid (coating agent) on a lens base material rotationally held and then spreading the coat liquid over the base material surface by rotating the base material and further throwing excess coat liquid by rapidly rotating the base material and thereby regulating the desired film thickness. The spin coat method has been widely employed as a coating agent application process in forming a hard coat layer on the lens surface because of having the operation simplicity.
The case of using a photo-chromic coating agent with the spin coat method has the following specialties as compared with conventional spin coat methods.
Firstly, since it is necessary to give sufficient photo-chromic properties to a base material lens only by a photochromic coat layer in the coating method, the film thickness is necessary to be thick so as to be from 5 to 100 μm. In order to easily form the coating film having such a thickness, the photochromic coating agent is regulated to have a high viscosity of for example, a viscosity at 25° C. of from 20 to 1000 centipoises and also it is spread at a low rotational speed (as compared with the case of using conventional coating agents for hard coat) of, for example, not more than 1000 rpm.
Secondarily, the lens base material usually contains an ultraviolet absorbent. In the case that light transmitted through such a lens (light in which ultraviolet ray is decreased) is irradiated on the photochromic coat layer, it is difficult for the photochromic compound to develop coloring. Accordingly, in the case of preparing a photochromic lens with the above “coating method”, it is necessary to form the photochromic coat layer on the surface of the lens front surface side (the surface opposite to the surface faced to the eyes when glasses are worn), usually the convexly curved surface.
However, when the photochromic coat liquid having a high viscosity is applied with spin coat on the lens surface, excess coat liquid, which has not been thrown off at the time of rapid rotation, stays in the periphery of the base material so that the thickness of the periphery of the photochromic coat layer is thicker than those of other parts (thickening of periphery) to cause problems such that the unevenness is induced in the thickness of the coat layer and the optical properties of the lens are lowered. This remaining liquid can fairly be decreased by removing the liquid staying in periphery of the base material by means of a jig such as a spatula or the like at the time of spin coat, but the thickening of the periphery cannot be dissolved completely.
In the case of lenses for spectacles, a lens subjected to coating is processed so as to fit the flame shape and then used. Therefore, since the periphery part where thickness is large (periphery thick part) is cut out in the processing, the lenses for spectacles practically do not have the above problems in many cases. However, the applications using lenses which are not subjected to cut processing still have the above problems. Furthermore even in the application of lenses for spectacles, when lenses having a small diameter are subjected to coating, the area of the thick periphery part becomes relatively large and the influence thereof cannot be ignored. Moreover, when flat-like lenses having a small curvature are subjected to coating, it has been found that the region of the thick periphery part is spread (occasionally, the region about 10 mm apart from the periphery in the radius direction becomes a thick periphery film part). In order to apply “the coating method” to lens base materials having various forms, it is necessary to further narrow the region of the thick periphery film part.
Under the circumstances, it is an object of the present invention to provide a process which does not cause periphery thickening when preparing products having a thick coating layer with a thickness, for example, from 5 to 100 μm, by spin coating method comprising applying a coating agent having a high viscosity such as a photochromic coating agent on the surface of a base material and then curing.
As described above, because the coating agent flows after the coating, the coating layer having a uniform thickness sometimes cannot be prepared. This problem will be probably solved by quickly curing the coating film after the coating film forming. But, in the case of carrying out the curing step in an atmosphere of an inert gas, the uniformity of the coating film thickness is occasionally deteriorated because it takes a certain time for gas purging. Therefore, in consideration of industrial preparation, the time for maintaining the uniformity of the coating film thickness is preferably longer.
As described above, it is another object of the present invention to provide a technique capable of maintaining the uniformity of the coating film thickness over a longer period of time after the coating film formation.
The present inventors have earnestly studied in order to solve the above objects. As a result, the present inventors found that during the time from formation of a coating agent-coated film with the spin coating method to curing thereof with light irradiation, the time of maintaining the uniformity of the coated film thickness can be prolonged by maintaining the base material at a low temperature, and also found that during this time, by rotating the base material at a specific rotational speed, the thick periphery film part is narrowed and the time capable of maintaining the uniformity of the coated film thickness can be more prolonged. Thus, the present invention has been accomplished.